It is a common occurrence for there to be a requirement to observe a scene which may contain one or more sources bright enough to damage or temporarily blind eyes or an artificial optical sensor. Workers in many scientific or industrial environments are often subject to hazardous exposure to intense light from optical lasers, arc welding equipment, or other sources. Protective equipment often decreases the worker's ability to perceive his environment and thereby decreases his productivity and increases his danger from other hazards in the environment. A particularly common example is that of manual arc welding, where the user's eyes must be protected from the excessively bright arc, and yet at the same time the user must be allowed to see the material he is working with.
Similarly, the presence of a hostile laser is a potential danger to present and future military operations. Many military systems rely on sensitive optical detectors or on the human eye enhanced by light gathering optics. The very sensitivity of the detector and the light gathering power of the optics renders the eye or the detector vulnerable to damage from hostile laser light. Methods of protection must address both the prevention of damage or injury and the continuing function of an optical system when it is illuminated by a threat laser.
Current methods of dealing with very bright sources in a field of view have serious problems and limitations. In manual arc welding, for example, the welder positions a hood with a light absorbing window in front of his eyes for protection from the excessively bright arc. The window provides a uniform attenuation which severely reduces or eliminates viewing of the work and surroundings until the arc is struck, and continues to restrict the field of view after the arc is struck.
In the past few years, electro-optically switched windows for use as eye protection during arc welding have been developed which are clear until an arc is struck and become uniformly absorbing within microseconds to milliseconds after light from the arc is detected. U.S. Pat. No. Re. 29,684 is an example of such a protective device. It relies on a photodetector to detect the lighting of the arc, a circuit to apply a voltage across the device when the lighting of the arc is detected, and a layer of liquid crystal material sandwiched between a pair of polarizers so arranged that when a suitable electrical potential is established the assembly will change from a uniform light transmitting condition to a uniform largely absorbing condition. Since such a switching process is "active"; power sources, light detectors, and electronic circuits are required. This adds to the cost and complexity of the window. And, as in the case of the absorbing window of a simple welding hood, the field of view of the welder is restricted because uniform attenuation of the light does nothing to reduce the contrast in apparent brightness between the arc and the remainder of the field of view.
Measures for the protection of eyes and sensors against lasers include the use of one or more fixed optical filters capable of excluding narrow spectral lines which may be emitted by the most obvious laser threats. However, this approach is not suitable for use against agile military lasers, which can operate at various wavelengths of the optical spectrum. The effectiveness of fixed interference filters is also limited because their optical density at a threat wavelength may be greatly reduced when light is incident from angles different from a specific design angle; these filters change the apparent color of a scene; and as the number of filtered lines and the optical density of a filter increases, the scene apparent brightness decreases, reducing visibility.
Various liquids and solids exist, including solutions or suspensions of limiting materials, which when placed near or at an intermediate focal surface of an optical system will transmit scenes to a final focal surface with little distortion and attenuation when the scene contains no excessively bright source, but which will very substantially attenuate the intensity occurring at a final focal surface (at a detector or eye of an observer) when an excessively bright source is encountered. The attenuation may have a nonlinear dependence on intensity and/or fluence and may be a combined effect of absorption, reflection, scattering and refractive defocusing. This approach is most effective for short pulse lasers; when lasers emit continuously, the scattered, reflected and defocuscd light may obscure the scene for an unacceptable period of time. In addition, the backscatter and reflection from these protective devices may be a problem in some applications.
Active electro-optical and mechanical devices can limit light intensity by switching the transmission state of an optical system temporally, spectrally or spatially. These devices are relatively expensive and encumbering, and the broader their capability of dealing with a threat the more encumbering they may be to the user.
In view of the foregoing, there is a continuing need for a simple, low-cost, light-weight, passive device which permits an optical sensor to better maintain its desired function by reducing the apparent brightness of a very bright source in an otherwise moderately bright field of view, which provides protection for eyes or artificial optical sensors against bright sources in the field of view over the entire optical wavelength band, which does not change the color of a scene, which is not a strong retroreflector of bright sources, and which does not block ordinary vision or obscure the field of view with excessive unwanted scattered or refracted light. The present invention satisfies such needs.